Today, various prepaid cards, each prestoring money amount information corresponding to a certain amount of money paid in advance, are being widely used in various commercial transactions, such as for using railroad facilities and public telephones and playing with Japanese pinball (hereinafter “pachinko”) game machines. These prepaid cards are inserted into card reader/writers attached to or contained in various pieces of equipment, such as automatic ticket checkers, automatic ticket vending machines, public telephones and game machines, where each amount due is subtracted from the remaining balance on the card and the prestored money amount information is rewritten accordingly.
Besides, in various banking agencies and the like, account transfer services using personal computers and public telephone lines have come into wide use, and it is expected that every banking and currency settlement service will be conducted through an electronic currency system in the near future (as disclosed in, for example, Japanese Patent Publication No. HEI-7-11723).
Recently, an increasing number of persons have been attempting to tamper or copy the stored data on the prepaid cards without due authorization, so that prepaid card issuing companies are exercising, against such fraudulent attempts, preventive measures that include encryption and scrambling of the stored data. In the account transfer and various electronic business transactions, many persons have been attempting to acquire other person's authentication data in a fraudulent manner, in order to make unfair benefits by pretending to be the true prepaid card holder.